I've been looking into this as I've just picked up an 03 ACE.
Saddlemen makes one that comes in under $450. Corbin also makes one, more expensive at around $700. Given my past experience with Corbin I'd put that as the gold standard, but the Saddlemen is a very good option. I've never been crazy about Mustang saddles. Saddle choice is such a personal thing, though, so your mileage will vary.
I have Mustang seats on both the Spirit and my Valkyrie (I have two sets for the Valkyrie). Even though I paid full pop for the seats on the Spirit, I was able to find a driver and pillion seat set for the Valkyrie at half price on the used market. Got lucky, the previous owner rode on the seats once or twice and decided they didn't like how the Mustang seat raised him up a bit. Said his feet could't touch the ground.
The second Mustang seat I found for the Valkyrie was only $190 brand new with the back rest! By chance I happened upon a closeout sale and bought it right away. I use that seat for a solo setup.
Anyway, with some patience and perhaps a bit of luck, you may be able to find the seat you want at a price that's sane.
Well, there is also what is known as a "Local Option".
Find a local upholsterer, check and see if they've done motorcycle seats before.
It is all in the Foam...as to how a seat feels, how well it holds up.
The seat companies have pretty much done all the hard research,
which is why they cost so much.
On the upside, a local shop can give you nice patterns, multiple colors of leather,
personalize the heck out of it. Saw a Shadow around Panama City that had
a Red Flame sewn into the seat, Wicked!